1. Field of the Invention
This invention relates generally to optical signal processing and more particularly to an optical multiplexing apparatus employing pseudorandom bit sequence (PRBS) mode modulation.
2. Description of the Related Art
Optical networks using wavelength-division-multiplexing (WDM) and Dense WDM (DWDM) techniques have been installed throughout the world in response to increasing demand for communications channel capacity. Recent advances in WDM and DWDM technology have focused on improving capacity with either smaller channel wavelength spacing or wider wavelength range. Channel spacing is limited by many factors, including optical filter efficacy, frequency drift, interferometric crosstalk, fiber dispersion and nonlinearity. The art is replete with various multiplexing proposals for obtaining additional capacity in existing optical fiber systems.
Some early practitioners proposed exploiting available WDM bandwidth by using a polarization-division-multiplexing (PDM) scheme in which independent WDM channel signals are simultaneously transmitted on orthogonal polarizations to reduce interchannel crosstalk and improve the operation of optical filtering of adjacent channels. For example, Hill et al. (“Optical Polarization Division Multiplexing at 4 Gb/s.” IEEE Photon. Technol. Lett.,” Vol. 4, No. 5, pp. 500-502, May 1992) proposes challenging the speed bottleneck in the electronic circuit components used for time-division multiplexing operations by using a simple PDM system for doubling channel capacity by simultaneously transmitting two independent data sets as two optical signals having separate optical states of polarization (SOPs). Hill et al. distinguish PDM from the polarization shift keying (POLSK) technique used to transmit the bits from a single-word generator in any of two or more polarization states. The Hill et al. system uses simple coherent heterodyne detection to demultiplex the two signals and is admittedly impractical without additional (unspecified) polarization control techniques to compensate for the effects of PMD in fibers longer than a few thousand meters.
Generally, there is now an accepted understanding in the art that simple PDM in optical fibers longer than a few thousand meters is practical only for soliton (a solitary wave with nearly lossless propagation) trains because of polarization mode dispersion (PMD). In an early paper, Evangelides et al. (“Polarization Multiplexing with Solitons,” IEEE J. Lightwave Technol., Vol. 10, No. 1, pp. 28-35, January 1992) showed that solitons launched into a fiber with orthogonal polarization may be demultiplexed at the output (so long as crosstalk is avoided by ensuring the solitons do not overlap in time) because the common transit history of the solitons imposes identical polarization errors on each, thereby ensuring that the relative polarization orthogonality is undisturbed by any amount of PDM encountered in the fiber, even over distances of thousands of kilometers. Evangelides et al. note that the polarization channel separation possible with solitons is not possible with other pulses. Later, Ono et al. (“Polarization Control Method for Suppressing Polarization Mode Dispersion Influence in Optical Transmission Systems,” IEEE J. Lightwave Technol., Vol. 12, No. 5, pp. 891-8, May 1994)
Indeed, because the combination of polarization-dependent loss (PDL), polarization dependent gain (PDG) and PMD all contribute to fading in WDM systems, most existing WDM systems employ polarization scrambling to minimize unwanted fading, thereby teaching against any use of PDM for increased channel capacity. For example, in U.S. Pat. No. 6,137,925, Stimple et al. disclose a multi-wavelength polarization scrambling device intended to minimize the correlation of signal polarization in a WDM channel.
Nevertheless, some practitioners proposed using PDM with soliton trains to overcome specific problems unrelated to optical fiber channel capacity. For instance, in U.S. Pat. No. 6,188,768 B1, Bethune et al. disclose an autocompensating quantum cryptographic key distribution system based on using soliton trains with PDM to ensure the impossibility of accurate eavesdropping. Others propose using PDM in free-space optical communications systems not subject to significant PMD. For example, Kuri et al. (“Multiple Polarization Modulation (MPLM) System for Coherent Optical Space Communication,” Global Telecommunications Conference, 1995. GLOBECOM '95., IEEE, Vol. 3, pp. 2003-2007, 1995) proposes a novel MPLM system for the simultaneous independent transmission of modulated subcarriers and baseband signals. Kuri et al individually modulate the polarization ellipticity angle and the polarization azimuth angle with the modulated subcarriers and baseband signals, respectively, to avoid phase noise and polarization axis mismatch at the receiver.
Recently, some practitioners have proposed using sophisticated variations of the basic PDM concept to improve particular features of optical fiber channel performance. For example, in U.S. Pat. No. 5,900,957, Van Der Tol discloses an optical packet switching system that encodes the data and address information in two orthogonally-polarized signals that may be easily separated using passive optical devices. Van Der Tol observes that glass fibers usually do not maintain polarization over kilometer distances and therefore suggest several features intended to protect the relative orthogonality of the two polarized signals over long distances, a feature reminiscent of the earlier soliton train PDM systems. In another example, Hayee et al. (Summaries of Papers Presented at the Conference on Lasers and Electro-Optics, 1999. CLEO '99. pp, 181-182, 1999) describe a method for multiplexing two orthogonal polarizations of the same wavelength in a power ratio of two-to-one to partially overcome the well-known impracticality of PDM over kilometer distances because of random variations in fiber birefringence. By decorrelating the two signals in time, unbalancing them in power, and operating the modulators only in binary mode to exploit its full extinction ratio, Hayee et al. manage to squeeze enough improvement out of the PDM technique to demonstrate useful performance over a 95-km fiber. In yet another example, Zheng et al. (“Suppression of Interferometric Crosstalk and ASE Noise Using a Polarization Multiplexing Technique and a SOA,” IEEE Photon. Technol. Lett, Vol. 12, No. 8, pp. 1091-1093, August 2000) propose a PDM technique for overcoming amplified spontaneous emission (ASE) noise from optical amplifiers and crosstalk at the signal wavelength. Using a semiconductor optical amplifier (SOA), Zheng et al. multiplex an optical signal and its inverse as two orthogonally-polarized signals, the amplitudes of which add to a fixed value of logical one. Transmitting the two signals from the same SOA results in a fixed saturated output power level that solves the ASE and interference problems (both are significantly suppressed by the saturated SOA). The original signal is demultiplexed at the receiver with a polarization beam splitter (PBS) but Zheng et al. do not consider operation over fibers longer than 45 km. Similarly, Srivastava et al. (“A Polarization Multiplexing Technique to Mitigate WDM Crosstalk in SOAs,” IEEE Photon. Technol. Lett, Vol. 12, No. 10, pp. 1415-6, October 2000) suggests using polarization multiplexing to overcoming the effects of the crosstalk arising from SOA gain saturation. Two orthogonally-polarized optical signals are modulated with the data stream and its complement before being combined to from a signal having a constant average power without bit transition patterns. The two wavelength channels are then decorrelated by sending through a 10-km single-mode fiber to introduce a delay between the two channels. They report an additional 1 dB bit error rate (BER) power penalty because of the accumulated dispersion through the decorrelation and transmission fiber sections but do not discuss PMD or polarization dispersion loss (PDL).
As may be readily appreciated from these examples, there is a clearly-felt need in the art for a modulation method that improves the capacity of an optical channel subject to random fluctuations in fiber birefringence over long distances. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.